European Published Patent Application No. 0 477 400 describes an hydraulic coupler for a piezoelectric actuator in which the actuator transmits a lifting force to a master piston. The master piston is in force-locking connection to a guide cylinder for a slave piston. The slave piston, the guide cylinder and the master piston sealing the guide cylinder form an hydraulic chamber. A spring which presses apart the master piston and the slave piston is situated in the hydraulic chamber. Arranged around an end section of the guide cylinder and the slave piston is a rubber sleeve which seals a holding chamber for a viscous hydraulic fluid from a fuel chamber. The viscosity of the hydraulic fluid is adapted to the ring gap between the slave piston and the guide cylinder.
The slave piston mechanically transmits a lifting movement to a valve needle, for instance. In response to the actuator transmitting a lifting movement to the master piston and the guide cylinder, this lifting movement is transmitted to the slave piston by the pressure of the hydraulic fluid in the hydraulic chamber, because the hydraulic fluid in the hydraulic chamber is not compressible and during the short duration of a lift only a small portion of the hydraulic fluid is able to escape through the ring gap into the storage chamber formed by the rubber sleeve. In the rest phase, when the actuator does not exert any pressure on the master piston, the spring pushes the slave piston out of the guide cylinder and, due to the generated vacuum pressure, the hydraulic fluid enters the hydraulic chamber via the ring gap and refills it. In this way, the coupler automatically adapts to longitudinal expansions and pressure-related extensions of a fuel injector.
What is disadvantageous in the related art is that the sealing provided by a rubber sleeve, which is usually pressed against the end section of the guide cylinder and the slave piston by two clamping rings, is unsatisfactory in the long term. It is possible that the highly viscous hydraulic fluid and the fuel mix and the coupler breaks down. When fuel, such as gasoline, reaches the interior of the coupler, a loss of function may occur since this fluid, due to the low viscosity of gasoline, may flow too rapidly through the ring gap and no pressure is able to be generated in the pressure chamber during the lift duration.
The known related art also does not offer a solution for protecting the piezoactuator from contact with fuel, especially gasoline.
German Patent No. 43 06 073 describes a fuel injector having a piezoactuator which is to connected to a pressure piston having a large surface. This pressure piston is prestressed with respect to the piezoelectric actuator by a disk spring which is braced against the valve body of a fuel injector. The pressure piston is guided in a bore of the valve body and has a central bore hole in which a slave piston is guided, the slave piston being connected to a valve needle. Situated in the bore of the pressure piston, between the base of the bore and the slave piston, is a spring which provides an initial stress to the slave piston in the direction of a valve seat and pushes it out of the bore. The fuel injector has a valve needle that opens to the inside. A pressure chamber is located between the fuel injector valve body and the pressure piston and the opposite side of the slave piston. The pressure chamber is in connection with the actuator chamber via the ring gap between the slave piston and the pressure piston, the bore in the pressure piston and a connecting bore. The actuator chamber is used as a holding chamber for an hydraulic fluid. When the piezoactuator is actuated in response to a voltage being applied, the pressure piston is moved in the direction of the valve seat. Due to the increased pressure of the hydraulic fluid in the pressure chamber, the slave piston is pressed into the bore into the pressure piston, counter to the pressure piston's direction of movement, thereby lifting a valve needle off from the valve seat.
Disadvantageous in this known related art is that it does not provide a solution for a fuel injector opening toward the outside. Furthernore, it is disadvantageous that no devices for the rapid refilling of the pressure chamber following its return to the rest position are provided. 30 Finally, the design consists of a plurality of parts and is complicated since a pressure piston which is guided in a precise bore in the fuel injector, in turn requires a precisely worked bore for the slave piston.